The present invention relates to the field of fiber optics and more specifically relates to improvements in connector components and methods for use in making connections in fiber optic systems, to loopback devices for use in testing optical devices, and to methods of using such devices.
Fiber optic communication systems send messages in the form of pulses of light along thin strands of transparent material, referred to as optical fibers. One common application for such systems is in carrying digital data between computers in a network or between portions of a large computer. In a typical system, a device referred to as an optical transmitter includes a laser that emits light. The intensity of the light is varied in accordance with the information to be sent. The emitted light is focused on an end of an optical fiber so that the light is transmitted along the fiber. At the other end of the fiber, the light is directed onto a photodetector, which transforms the light into an electrical signal. The electrical signal also varies in accordance with the information being sent. A xe2x80x9cduplexxe2x80x9d system typically uses two fibers in parallel, and has a transmitter and a receiver at each end of the system so that information can be sent in opposite directions along the two fibers. The transmitter and receiver at each end typically are combined in a single device referred to as a xe2x80x9ctransceiverxe2x80x9d. Optical communication systems can transmit data at rates many times faster than systems using electrical wires, and offer other advantages.
Typically, the optical fibers are provided in optical cables. The fibers themselves are covered by protective coatings or xe2x80x9csheathingxe2x80x9d. The cable includes one or more individual sheathed fiber optics, covered by an external jacket and may also include components for protecting the cable against physical strain. To set up an optical communications system, cables of this type are connected to optical devices such as transceivers and to one another in much the same way as electrical cables are connected to electronic devices and to one another to set up an electronic system. However, connecting an optical cable requires that the individual optical fibers be precisely aligned with the mating fibers or devices. The optical fibers commonly are as small as 0.125 mm (0.005 inches) in diameter. To connect two fibers end-to-end, the mating ends should be aligned with one another within a few microns, i.e., within hundred-thousandths of an inch, and should be butted against one another with essentially no gaps. Even slight deviation from these tolerances can cause appreciable loss of light transmitted along the fibers and degradation of the signal. Likewise, when an optical cable is connected to a transceiver or other device, the fibers must be precisely positioned relative to the optical elements of the device. Optical cables are provided with devices referred to as xe2x80x9cconnectorsxe2x80x9d which can be engaged with mating connectors on other cables, or with mating features on transceivers or other devices, to align the fibers with the required precision.
One type of connector that has been proposed is referred to in the industry as an MT-RJ connector. U.S. Pat. No. 5,926,596 depicts a typical MT-RJ connector. Reference is made to the ""596 patent without admission as to whether or not such patent constitutes prior art against the present invention. As shown in the ""596 patent, a typical MT-RJ connector includes an exterior housing which resembles the exterior housing of the common xe2x80x9cRJxe2x80x9d plug used to connect a home telephone to a wall outlet. The housing has a flexible catch on its exterior. A xe2x80x9cferrulexe2x80x9d is movably mounted within the housing at a forward end of the housing, so that a forward face of the ferrule is exposed to the exterior of the housing. A spring inside the housing urges the ferrule in the forward direction. The ferrule has a pair of fiber bores for receiving two individual fibers of the cable, and a pair of pin holes for receiving alignment pins. A xe2x80x9cmalexe2x80x9d MT-RJ connector has alignment pins permanently disposed in its alignment pin holes, whereas a xe2x80x9cfemalexe2x80x9d MT-RJ connector has empty pin holes. The connectors may be permanently installed on the ends of fiber optic cables by the cable manufacturer. The cable manufacturer positions the fibers in the fiber bores and polishes the ends of the fiber precisely flush with the front of the ferrule.
To connect two cables end-to-end, male and female connectors are inserted into opposite ends of a hollow double-ended socket so that the catches on their housing engage with the socket and the socket physically holds the housings in crude alignment with one another. The pins on the ferrule of the male connector engage the pin holes in the ferrule of the female connector, and hold the ferrules, and hence the fibers, in precise alignment with one another. The springs in the housings urge the ferrules forwardly so that the front faces of the ferrules, and hence the ends of the fibers, abut one another. Devices such as transceivers are equipped with single-ended sockets adapted to receive the housing of a connector. Such sockets are equipped with pins corresponding to the pins of a male MT-RJ connector for engaging the ferrule of a female connector so as to hold the ferrule and hence the fibers of the cable in precise alignment with the device.
Despite considerable effort devoted by the art to development of fiber optic connectors, sockets and related components, there are still needs for further improvements.
There exists a need for further improvement to create more economical connectors. Anything which can be done to eliminate parts and assembly expense in the connector and in the process for attaching the connector to the cable at the factory would be desirable.
It would be desirable to provide a field-installable connector which can be attached to a raw, newly cut cable end by a technician in the field. Such a connector should be compatible with the factory-prepared connectors and with the sockets used for such connectors. Moreover, such a field-installable connector should fit within the space available for installation according to industry standards. Such a connector should be relatively easy for the technician to install and should provide a good optical connection.
There are also needs for improvement in loopback testing of optical transceivers. As well known in the art, many optoelectronic components such as network hubs and interfacing devices incorporate numerous optical transceivers. Each transceiver has a transmitter arranged to send optical signals and a receiver arranged to receive the optical signals and convert the same back to electronic signals.
The transceivers commonly are tested by a xe2x80x9cloopbackxe2x80x9d test. In a loopback test, a single fiber is connected to the transmitter and receiver of a single transceiver so that light sent by the transmitter is received by the receiver. If the transceiver can send signals to itself this manner, then both the transmitter and receiver incorporated in the transceiver are operational. Such a test can be conducted by connecting both ends of a single fiber to a standard MTRJ connector to form a local loop and inserting that connector into the socket associated with a transceiver. Several problems are encountered using this approach. The optical power appearing at the receiver may be too great for the receiver to handle. Typically, the transmitter is designed to send an optical signal strong enough to propagate over tens, hundreds or thousands of meters of fiber, whereas the fiber in a local loop may be a meter or less in length. Thus, the signal reaching the receiver from the transmitter over the local loop is far stronger than the receiver can accommodate. Also, where numerous transceivers incorporated in a single assembly are to be tested, connectors with such local loops must be inserted into sockets associated with all of these transceivers. All of these loops form a tangle of fibers overlying the surface of the assembly and making it difficult for the technician to work with the device. Moreover, the local loops, with their associated connectors, are relatively costly.
The present invention addresses these needs and provides improvements that can be used in MT-RJ connectors, couplers and loopback systems. The improvements are described below.
In one aspect of the present invention, a male MT-RJ connector includes a housing adapted to fit within an MT-RJ socket, a ferrule mounted in the housing in substantially fixed position, the ferrule having a front face and having fiber bores extending to said front face, and alignment pins projecting from the ferrule. Such a connector is similar to a normal MT-RJ connector, but has its ferrule fixed to the housing rather than movable with respect to the housing. This aspect of the present invention incorporates the realization that because male MT-RJ connectors are not normally connected with devices such as transceivers but instead are mated only with female MT-RJ connectors which include movable ferrules, and the further realization that movability of the ferrule with respect to the housing in one of the two mating elements is sufficient to permit alignment of the mating ferrules. Thus the elements needed to make the ferrule movable can be omitted in the male connector while still providing a usable male connector.
Indeed, the preferred connectors according to this aspect of the invention can provide results superior to those achieved with more expensive male connectors having floating ferrules. When two connectors are mated with one another in a coupler, the pins of the ferrule of the male connector may initially be misaligned with the pin-receiving holes in the ferrule of the female connector. If this initial misalignment is within the design limit, typically less than one-half the diameter of a pin, the pins will find the holes and guide the ferrules into precise alignment with one another. However, if the initial misalignment is greater than this limit, the pins will not enter the holes and the connection cannot be made. This condition is referred to in the art as xe2x80x9cstubbingxe2x80x9d. There is some initial misalignment due to tolerances on the coupler and the connector housings. Where both connectors are made with xe2x80x9cfloatingxe2x80x9d ferrules, movable relative to their respective connector housings, the float may contribute additional initial misalignment, up to two times the amount of float or movability provided in each connector. This can occur if both ferrules are displaced in opposite lateral or vertical directions relative to their connector housings. However, where only one connector can float, the additional initial misalignment due to float is at most one times the amount of float in one connector. Thus, the male connectors according to this aspect of the invention, with fixed ferrules, reduce the possibility of stubbing and provide greater reliability.
A connector with a fixed ferrule a connector may be made as a field-installable connector. In this case, the connector further comprises a field termination unit having fiber bores therein, and pre-installed fiber segments extending from the field termination unit fiber bores and extending in the fiber bores of the ferrule to the front face of the ferrule. The field termination unit may be disposed in the housing immediately behind the ferrule. The connector further comprises a fitting retaining the field termination unit in the housing. The fitting may be a crimp nut having a rear portion defining a channel adapted to pass the fibers of a fiber optic cable. The rear portion of such a crimp nut is adapted to fit within the jacket and reinforcement of the cable so that the cable can be secured to the crimp nut by a crimp ring encircling the jacket.
More generally, a fixed-ferrule connector may include a device disposed in the housing and engaging the rear surface of the ferrule to maintain the ferrule in position within the housing. The pins have heads projecting from the rear of the ferrule. The device may engage the pins and may retain the pins against forward and rearward motion relative to the ferrule and housing.
A further aspect of the present invention provides a device for use in making a male MT-RJ connector. The device has a body adapted to fit within the housing of an MT-RJ connector. The body defines a forward wall adapted to engage the rear surface of a ferrule and a pair of pin head slots disposed rearwardly of the forward wall. The pin head slots are open to a top surface of the device so that a ferrule with pins therein can be advanced downwardly relative to the device to engage the heads of the pins in the pin head slots. Preferably, the body defines a generally U-shaped fiber channel open to the top of the device at the rear of the device.
In a related aspect of the present invention, a method of making a male MT-RJ connector comprises the following steps: (a) assembling the fibers of a fiber optic cable into fiber bores of a ferrule, securing the fibers to the ferrule and polishing the front face of the ferrule and the forward ends of the fibers; (b) assembling pins into the ferrule so that heads on the pins project rearwardly from the ferrule, and so that the pins project forwardly from the front face of the ferrule; (c) advancing the ferrule and fibers downwardly into a device so as to engage the heads of the pins in pin head grooves of the device and place the fibers into the device so that the fibers extend from the rear of the device and thereby form a subassembly; and (d) placing the subassembly formed in step (c) into a housing so that the device is engaged in the housing and abuts the rear surface of the ferrule to hold the ferrule in position in the housing.
Preferably, the method further comprises the step of securing the jacket of the cable, a reinforcement of the cable or both to the rear end of the device. The step of placing the subassembly into the housing includes advancing the subassembly forwardly to engage features of the device and housing and thereby lock the device to the housing.
A further aspect of the present invention provides a loopback test unit for an optical transceiver. The transceiver has input and output fibers and a connector adapted to mate with a fiber optic connector on a fiber optic cable so that the input and output ports of the transceiver are in optical communication with fibers of the cable. The loopback test unit includes a housing having exterior configuration corresponding to the configuration of a fiber optic connector housing on a fiber optic cable. The unit further includes an optical fiber. The fiber has two ends positioned relative to the housing so that both ends of the fiber will be disposed in optical communication with the input and output ports of the transceiver to place the output port of the transceiver in communication with the input port of the transceiver when the housing is mated with the connector of the transceiver. The test unit most preferably further includes a boot secured to the housing enclosing the fiber.
Preferably, the fiber is unsheathed, bent, or both. Additionally, it is preferred that the bent fiber is in the form of one or more generally circular loops enclosed within the boot. The loopback test unit according to this aspect of the invention thus provides a self-contained device which can be plugged into a connector on an optoelectronic component and used to test the component. Units of this type are useful in test of any device with a transceiver, but are especially valuable in use with devices such as a network hubs and interfacing devices incorporating numerous optical transceivers. Loopback test units according to this aspect of the invention eliminate the tangle of overlapping loops encountered with conventional test methods.
The optical fiber may be formed from a polymer or glass. Preferably, the optical fiber has a diameter larger than the nominal fiber diameter of the transceiver. That is, the fiber has a larger diameter than the fiber normally used with the transceiver, and hence the tolerances required in aligning the loopback test unit fibers with the transceiver are considerably looser than those required for alignment of a nominal fiber with the transceiver. As further explained below, such a fiber tends to attenuate the signal. This aspect of the present invention incorporates the realization that such attenuation, while undesirable in a normal communication fiber, is actually desirable in a loopback test unit fiber to prevent overloading the receiver.
The connector of the transceiver typically is adapted to engage and disengage the connector by mating and demating motion of the connector in forward and rearward directions. The boot and housing desirably have dimensions transverse to the forward and rearward directions approximately equal to the corresponding dimensions of a fiber optic connector or smaller. Where the transceiver is arranged to mate with and MT-RJ connector, the boot has vertical dimensions of about 11-mm or less and lateral dimensions about 10.5-mm or less. Preferably, the boot is mounted in fixed position relative to the housing.
The test unit typically comprises a ferrule mounted on the housing of the test unit. The optical fiber has ends mounted to the ferrule and exposed at a front face of the ferrule. Preferably, the ferrule is formed by molding a polymeric composition. Such a molded ferrule can meet the relatively loose tolerances on fiber alignment associated with the loopback test unit. The use of such a molded ferrule considerably reduces the cost of the test unit.
According to a related aspect of the invention, an optical communication device includes a plurality of optical transceivers and a plurality of connectors optically coupled to the transceivers. The connectors are disposed side by side in an array, and a plurality of test units as described above are releasably connected to the connectors for testing the transceivers.
A further related aspect of the invention provides a method of testing a plurality of transceivers in an optical communications device comprising the following steps: providing a plurality of optical test units as described above; temporarily engaged with an array of connectors associated with the transceivers; testing the transceivers by sending signals through the test units and then removing the test units. Preferably, the step of providing a plurality of test units is performed by installing the test units at a manufacturing location, and shipping the transceivers, connectors and test units together to a usage location. The step of testing the transceivers is performed at the usage location. Desirably, the loopback test units are so economical that they may be discarded after the test is complete.